(1) Field of the Art
This invention relates to a method for subjecting, for example, a semiconductor wafer to exposure through a photomask bearing a pattern drawn thereon by making use of UV light.
(2) Description of the Prior Art
Upon fabrication of semiconductor devices such as integrated circuits (ICs), large-scale integrated (LSI) circuits and very-large-scale integrated (VLSI) circuits, and the like, photoetching is carried out, for example, to remove parts of silicon dioxide layers provided on the surfaces of substrates formed of silicon wafers in accordance with desired patterns such as circuit patterns. This photoetching process generally requires such steps as forming a photoresist film on a silicon dioxide layer on a silicon substrate and then transferring mask features to the photoresist film by UV light. Subsequent to the exposure, the photoresist film is developed and the silicon dioxide film is subjected to an etching treatment. Thereafter, a further processing for the formation of a circuit such as diffusion or ion implantation is applied to the silicon substrate through the etched and exposed areas of the silicon dioxide layer.
A semiconductor wafer is usually circular. Its surface area is divided into minute sites arrayed in rows and columns. These minute sites are eventually cut from one another into chips which make up individual semiconductor devices. The diameters of semiconductor wafers are usually 3 inches, 5 inches, 6 inches or so. They however tend to become larger as their fabrication technique advances.
The resist-coated surface of a semiconductor wafer may be exposed at once in its entirety to light so as to print pattern images on all the numerous minute sites thereof at the same time. This exposure method is however accompanied by such problems that it requires a large-output mercury vapor lamp, thereby making the exposure system inevitably large, that technique of a considerably high level is also indispensable to ensure uniform illuminance on the whole exposed area of each semiconductor wafer, and that a semiconductor wafer becomes more susceptible to warping and the focusing operation of a projected image thus becomes more difficult as the semiconductor wafer becomes larger. Consequently, the above exposure method has difficulty in meeting the recent trend toward larger semiconductor wafers.
With the foregoing in view, the so-called stepwise exposure method has recently been proposed. According to the stepwise exposure method, the minute sites arrayed in rows and columns on the resist-coated surface of a semiconductor wafer are successively exposed one by one so that patterns are successively printed on the minute sites. This stepwise exposure method has brought about such significant advantages that inter alia, it permits use of a small-output mercury vapor lamp and hence a small exposure system because it is necessary to expose only a small area equivalent to one of the minute sites in each exposure, uniform illuminance can be readily achieved on the exposed area of the semiconductor wafer because the sites subjected respectively to successive exposure operation are small, and the focusing of projected images can be attained with ease in every exposure operation since the site exposed in each exposure operation is small and is less affected by the warping of the semiconductor wafer. Consequently, the stepwise exposure method permits with high accuracy the printing of patterns.
It is simple and advantageous to use a reducing and projecting lens system as an exposing optical system for printing a reduced image of a mask pattern on a semiconductor wafer. This reducing and projecting lens system is used by disposing it in the optical path which extends from an exposing light source to the semiconductor wafer. Conventionally-employed reducing and projecting lens systems generally have high transmittances for light of 436 nm. Reflecting this, supervoltage mercury vapor lamps capable of radiating light whose peak wavelength is also 436 nm have also been used as exposing light sources.
Reflecting the ever-increasing demand toward semiconductor devices of still higher integration degrees in recent years, it is necessary to make linewidths of patterns to be printed still finer and hence to improve the resolutions of exposure systems still further. In conventionally-employed supervoltage mercury vapor lamps, the wavelengths of the maximum peaks of their radiant lights are primarily 436 nm so that they match the light transmittances of reducing and projecting lens systems which have conventionally been employed. When radiant light of such characteristics is used for exposure, the minimum line width which can be resolved is limited to 1 .mu.m or so. In order to improve the resolution still further, it is indispensable to make the wavelength of the maximum peak of radiant light still shorter.
However, no reducing and projecting lens system capable of permitting efficient transmission of short-wavelength light shorter than 436 nm has been available until lately. Fo the lack of such lens system, an exposure method has been developed to expose at once the entire exposure area of a semiconductor wafer by using an optical system which was constructed to reflect short-wavelength light efficiently by combining mirrors without relying upon any lens system. Such a method is however accompanied by a problem that the structure of the optical system becomes unavoidably complex.
Very lately, lenses permitting efficient transmission of short-wavelength light of 365 nm have been developed. Under the circumstances, it has become feasible to improve the resolution by using a reducing and projecting lens system. However, conventional supervoltage mercury vapor lamps which have been used as light sources for exposure were too low in the radiation efficiency of short-wavelength light of 365 nm and were not able to achieve efficient exposure.